This invention relates to a method and apparatus for unloading the burden from a tow comprised of breast-tied strings of barges moored initially at an access platform where strings of barges can be formed with forward and aft soft ties to compensate for barge draft changes as the burden is removed in a floating unloading slip by two serially-arranged unloaders, preferably excavators. The burden is unloaded by initially removing an upper layer comprised of about 60% of the burden with the first of the unloaders and then the remaining layer of burden is removed by the remaining unloader. The last barge of the first string can be softly tied to the forward barge in a second string which is provided with soft ties at the access platform. As the first string of barges passes from the slip, they are separated from the second string and moved to an adjacent fleeting area where hard ties are again established between the barges of the first string as well as second and third strings of barges after being unloaded in the slip and placed abreast thereto.
As is known in the art, barges comprising a tow are usually made up of three strings, each comprised of five barges that are latched together in a forward-to-aft end relation with a wire rope, chain, etc. so that substantially no relative movement can occur between the individual barges in the string. The barges in the strings are latched together by hard-breast ties so that the barges in the strings form five tiers of three side-by-side barges. The connections between the barge tiers and strings are hard ties formed by wrapping steel cables, chains, etc. tightly between capstans by the use of a tensioning device such as a ratchet or winch.
In the known procedure, a tow is generally moored at a landing station where the hard ties are removed and each barge is moved to the location of an unloader where the barge is passed forwardly and backwardly beneath a ladder-type unloader or a continuous bucket elevator to unload the burden. A towboat or a barge haul is used to control back and forth movement of the barge at the unloader. The unloader is operated to remove the burden in layers. The first layer is removed from the barge in a first pass beneath the unloader. The towboat or barge haul is then operated to move the barge backwards whereupon the unloader is again positioned and operated to remove a further layer of burden. The cost for unloading a barge is of utmost importance; however the unloading operation is, as a practical matter, carried out by a two-pass operation. After each barge is unloaded, it is advanced to a clean-up area where residual amounts of burden are cleaned from the barge. Typically, for example, when a barge carries a burden of about 1500 tons, there is usually between 10 to 50 tons of residual burden left in the barge after unloading by the ladder or bucket unloader. The clean-up operation in each barge is a costly, time-consuming operation. Demurrage charges can be imposed for excessive unloading times. The clean-up operation is usually carried out by loading an appropriately-sized, self-contained scraper into the barge at the clean-up station and used in conjunction with a clamshell unloader to remove residual burden. The burden is either transferred into another barge for future unloading or delivered to a land-based storage pile by a truck. After the barges are completely emptied, they are moved to a fleeting area where a tow of empty barges is assembled.
Another conventional practice is to move barges in a string back and forth for two or more unloading passes. This is undesirable, on the one hand, because excessive rope slack to the forward and aft loose ties permits violent impacting between the aft and forward ends of the barges in the string; while on the other hand, inordinate amounts of time is consumed to back up the entire string so that the length of one barge can again move forward for a subsequent unloading pass. Two or more barge-unloading passes are necessary to maintain stability of the barge, prevent possible sinking and avoid destructive loading on the barge itself. Most barges used in the waterways in the United States today are of massive structures but lack an adequate reinforcement which will prevent breaking in half a barge when one-half is loaded with burden and the remaining half is empty. Moreover, should this loaded condition occur, the unequal load distribution may cause the loaded end of the barge to fall below the water level whereby the barge will ultimately sink. Some barge constructions are designed to withstand the forces that will be imposed when one-half of the barge is empty while the remaining half of the barge is loaded. However, the attitude of such a barge in the water at the unloader renders movement of the barge erratic and difficult to control.
In a conventional barge-unloading arrangement, when a barge is received in the slip with an inordinate amount of water mixed with the burden, such as coal, special measures are required to pump the slurry to a treatment area to recover the coal and clean the contaminated water. The slurry is removed from the barge at the clean-up area which further extends the time required to unload the barge and return it, or an entire string of barges, to a fleeting area.
Sometimes barge-unloading operations must be curtailed due to large fluctuations to the water level in a body of water which also places serious design constraints on the unloading apparatus. For example, there is as much as a 36-foot water level fluctuation on an annual basis to the Mississippi River. The water velocity also imposes design contraints on the apparatus to such an extent that conventional ladder unloaders can only be utilized with large capital expenditures for heavyduty equipment and massive support structures.
To overcome these deficiencies, the present invention provides a barge-unloading system which is more efficient than barge-unloading systems known in the art and avoids their shortcomings and drawbacks.